Various proposals have been made in the past for electric-powered vehicles. To date, for a number of reasons, electric vehicle systems have yet to become generally commercially viable for urban and highway applications. There have been proposals to employ zinc-air batteries for urban vehicle propulsion. An example is the publication "Improved Slurry Zinc-Air Systems as Batteries for Urban Vehicle Propulsion, " by P. C. Foller, Journal of Applied Electrochemistry, Vol. 16, pp. 527-543 (1986).
Metal-air battery structures are described in the following publications: U.S. Pat. No. 4,842,963, entitled "Zinc Electrode and Rechargeable Zinc-Air Battery;" U.S. Pat. No. 4,147,839, entitled "Electrochemical Cell with Stirred Slurry," U.S. Pat. No. 4,908,281, entitled "Metal-Air Battery with Recirculating Electrolyte;" U.S. Pat. No. 3,847,671, entitled "Hydraulically-Refuelable Metal-Gas Depolarized Battery System;" U.S. Pat. No. 4,925,744, entitled "Primary Aluminum-Air Battery;" U.S. Pat. No. 3,716,413, entitled "Rechargeable Electrochemical Power Supply;" U.S. Pat. No. 4,925,744, entitled "Primary Aluminum-Air Battery."
In U.S. Pat. No. 3,592,698, entitled "Metal Fuel Battery with Fuel Suspended in Electrolyte," there is described inter alia a method for circulating an electrolyte/metal fuel powder mixture through batteries; U.S. Pat. No. 4,126,733, entitled "Electrochemical Generator Comprising an Electrode in the Form of a Suspension" relates to a similar subject, using a circulated suspension of inert cores coated with an electrochemically active material. In U.S. Pat. No. 4,341,847, entitled "Electrochemical Zinc-Oxygen Cell," there is described a method in which an electrolyte is circulated in the annular space between concentric electrodes.
Electrical energy storage systems are described in the following publications: U.S. Pat. No. 4,843,251, entitled "Energy Storage and Supply Battery with Recirculating Electrolyte;" "Energy on Call" by John A. Casazza, et al., IEEE Spectgrum, Jun. 1976, pp. 44-47; U.S. Pat. No. 4,275,310, entitled "Peak Power Generation;" U.S. Pat. No. 4,124,805, entitled "Pollution-Free Power Generating and Peak Power Load Shaving System;" U.S. Pat. No. 4,797,566, entitled "Energy Storing Apparatus."
Regeneration of spent zinc-containing alkaline electrolyte is described in a number of prior patents. For example, in U.S. Pat. No. 3,847,671 (mentioned above), whole spent electrolyte is subjected to electrolysis, when zinc deposited at the cathode is removed with a wiper blade. The thus-removed zinc is said to be substantially heavier than the electrolyte (35-40% KOH) and hence falls to the bottom of each cell. In a particular embodiment, the cathode and anode are specified as being made from copper (or silver-plated copper) and carbon, respectively. In U.S. Pat. No. 3,981,747, it is proposed to regenerate the spent zinc in an alkaline electrolyte by reaction with a strongly electropositive metal, such as magnesium or aluminum, which displaces the zinc. In U.S. Pat. No. 4,341,847 (also mentioned above), spent zinc in the alkaline electrolyte is regenerated either by reversing the current and plating zinc on the anode, or by mechanically replacing zinc oxide particles by active zinc particles.
Moreover, it is of importance in batteries containing zinc electrodes that the zinc should not be consumed by a reaction with aqueous electrolyte, especially alkaline electrolyte which generates hydrogen gas, which reaction merely corrodes the zinc and prevents its availability for producing electric current. A number of prior patents relate to this problem.
Thus, e.g., in U.S. Pat. No. 4,112,205, double salts containing both mercuric and quaternary ammonium ions are used as inhibitors in galvanic cells comprising zinc anodes, notably in Leclanche-type batteries, containing ammonium chloride/zinc chloride electrolyte; U.S. Pat. No. 3,945,849 employs quaternary ammonium halides as inhibitor for zinc anodes in similar primary cells. U.S. Pat. No. 4,195,120 teaches alkaline cells containing a predominantly zinc anode and, as a hydrogen evolution inhibitor, a surfactant which is an organic phosphate ester of the ethylene oxide adduct type. Metal oxide inhibitors for zinc (in practice ZnO) electrodes are described in U.S. Pat. No. 4,084,047, in which the inhibitors are mixed thoroughly with the ZnO; the inhibitors taught in this patent are binary combinations of oxides which exclude mercuric oxide, the latter being regarded as an ecologically unsatisfactory additive for the ZnO electrode. According to U.S. Pat. No. 4,084,047, it was known to mix or alloy the active zinc in zinc-zinc oxide anodes and their supporting grid (e.g., copper or silver structures) with 0.5-5.0 wt. % mercury or mercuric oxide.
It will also be appreciated by persons skilled in the art that amalgamation of zinc with mercury has been known for a very long time, and that it is carried out in neutral or more usually, in acid solution, e.g., by reacting zinc with mercury chloride in dilute hydrochloric acid.
In U.S. Pat. No. 5,228,958, there is described and claimed a process for the regeneration of an at least partially-spent slurry having a dissolved phase and an undissolved phase for use in metal-air batteries.
In U.S. Pat. No. 5,232,798 there is described and claimed a method for the inhibition of corrosion in particulate zinc.
The disclosures of all of the foregoing publications (including patents), including also the prior art described therein, are explicitly incorporated herein by reference.
It is an object of the present invention to make possible, from a practical point of view, the general commercial viability of zinc-alkaline batteries, more particularly zinc-air batteries, for use in electric vehicle propulsion and energy storage systems. A more specific object of the invention is to provide a process for preparing a zinc slurry for zinc-alkaline cells and/or regenerating a rechargeable slurry for use in zinc-air batteries. Such a slurry would be applicable, for example, in a mechanically refuelable zinc-air battery in which, following discharge, the spent zinc is replaced with regenerated zinc for the subsequent discharge.
One of the major problems which required solution to make said regeneration process commercially feasible is an effective way for removing deposits from a major surface of an electrode in an electrochemical bath.
More particularly, the process described and claimed in co-pending U.S. Ser. No. 08/088,543 inter alia comprises electrolyzing an admixture containing zinc, which has been at least partly oxidized, to an oxidation product selected from the group consisting of zinc oxide, zinc hydroxide and zincates in a cell with a corrosion-resistant anode and a non-zinc-adherent cathode such that the zinc which deposits on said cathode self-detaches or is removable by a method selected from brushing, scraping, vibrating, the use of liquid jets, either fixed or moving, and the use of electrical pulsing, until no more than a pre-selected amount of zinc remain in the solution. However, the method described in said application was that the cathode was transferred to a separate container every ten minutes, where the deposited zinc was removed by scraping with a plastic spatula and homogenized into a particulate structure by means of a revolving nylon brush, while a clean cathode was placed in the electrolytic bath to continue the zinc recovery process.
While such a method indeed works, it is not practical for mass regeneration of thousands of anodes for the cells of electric fuel batteries.
As mentioned hereinbefore, U.S. Pat. No. 3,847,671 relates to a hydraulically refuelable metal-gas depolarized battery system which inter alia purports to teach an electrode arrangement as described in column 9, lines 25-32 as follows:
Wipers 66 extend the height of each electrode and are wiped across the face of each negative electrode to remove the spongy zinc deposit therefrom as fast as it forms, thereby making room for more deposit without filing the cell. The dislodged powder is removed from the bottom of each cell periodically to a manifold (not shown), from which it is pumped into slurry tank 48.
An examination of FIG. 5 of said patent reveals that wipers 66 are in fact vertical triangular prisms, with one apex serving to move horizontally back and forth across the electrode surface.
The problem with such a configuration, however, is that it has now been found that the wedge shape of the prism directs liquid in said bath towards the line of contact between the scraper and the electrode, whereby the resulting hydrodynamic forces work against an effective scraping contact and action.